For a battery assembly having a plurality of cells, it is required for a protective device thereof to monitor a voltage of each cell in real time so as to determine the operation state of the battery assembly. If the voltage of the cell cannot be detected accurately and timely when the cell is abnormally connected with the protective device or is disconnected from the protective device, the protective device cannot accurately protect the battery assembly in a charging or discharging state, thus causing damage to the battery assembly or even causing danger. The defect of conventional protective devices is that the detection accuracy is low, which may cause damage to the battery assembly.